EFFECTS OF COMBINED NITROGEN, Rhizobium STRAIN AND SUBSTRATE ON A SUPERNODULATING MUTANT OF Phaseolus vulgaris L.

Nodulation, plant growth and seed yield of the supernodulating mutant Rico 32M2-BS and its parent, cv. OAC Rico (Phaseolus vulgaris L.) are described in relation to supply of combined N and inoculation with Rhizobium leguminosarum bv. phaseoli and R. fredii. Combined N depressed nodule numbers more severely in the mutant than in its parent line, OAC Rico, at the first sample 14 d after seeding, but at 35 d, combined N increased nodule numbers in both the mutant and OAC Rico, and increased nodule mass in the mutant. In the absence of N, AR per plant increased between 14 and 35 d but AR per gram nodule weight declined. In the +N treatment, AR per plant and per gram nodule was greatest at 28 d, but in other samples AR of the +N treatments tended to be lower than in −N treatments. Plant growth of both the mutant and OAC Rico was stimulated by combined N. Both the mutant and OAC Rico nodulated well and effectively with three pure strains of R. leguminosarum bv. phaseoli and with indigenous soil strains. They both nodulated ineffectively with three strains of R. fredii, but the mutant plants still expressed their supernodulating characteristic. In the field test, plant growth of the mutant and OAC Rico was similar at the three levels of N fertilizer. Nodule dry weight was greater in the mutant than in OAC Rico; nitrogen depressed nodule dry weight in both lines. Seed yield of OAC Rico was 1.5 times that of the mutant.Key words: Combined N, common bean, nitrogen fixation, nodulation mutants, Phaseolus, Rhizobium, supernodulation